I had a couple of scottish ales that sat in both primary and secondary for a month (total of 2 months each), and they both carbonated with no additional yeast. One was a 60/- and the other was a Wee-Heavy. Neither of them was cooled below ~55F before I bottled, though.

I'd say it depends on alcohol content and the amount of CO2 you want to put into the beer, though. For something like a strong Saison or Belgian Golden Strong, where you want 3-4 volumes of CO2, it might be necessary.